Lasers make erbium a cool customer

A material that gets colder when hit with a laser beam may sound odd, but scientists have found that adding a dash of the metal erbium to certain compounds can turn them into miniature refrigerators.

The team at the University of the Basque Country (EHU), Bilbao, Spain, believe this type of cooling may benefit high-tech applications such as optical telecommunications and bioimaging.

The cooling effect is caused by inducing erbium to eject more energy than it absorbs from the laser beam, an effect called anti-Stokes emission. This phenomenon occurs when the laser is tuned to an energy just less than that required to excite electrons in charged erbium ions (Er3+). The ions make up this small deficit by absorbing some vibrational energy from the neighbouring atoms. As the atoms calm their jiggling, the temperature of the surrounding material falls. The erbium ions then return to their original state by fluorescence, emitting the energy they have absorbed in a photon of light.

The EHU team, led by Joaquín Fernández, encapsulated Er3+ ions in two different materials: a crystal containing potassium, lead and chloride (KPb2Cl5), and a fluorochloride glass. Crucially, they were able to make these materials with exceptional purity, since trace contaminants would scatter the laser and heat the sample. When the erbium-doped samples were hit with the laser, they saw that the crystal and the glass cooled by 0.5 and 0.7 °C respectively, they report in Physical Review Letters.1

'The main advantage of using erbium is that there are already laser diodes that operate at the frequencies and powers adequate for cooling,' co-author Ángel García Adeva told Chemistry World. Erbium is also commonly used to amplify optical signals, 'so one could cool the fiber at the same time that it amplifies the signal, which would allow use of higher laser powers,' he added.

'It's exciting that erbium in particular can be cooled,' agreed Mansoor Sheik-Bahae of University of New Mexico, Albuquerque, US. 'This shows great promise.'

Carl Mungan of the US Naval Academy, Annapolis, Maryland, US,discovered the same effect in the metals ytterbium and thulium a decade ago. He believes the work is 'potentially significant', but that the erbium ions must be tested with higher-power laser beams to assess how useful they will be in future applications.